The operational efficiency of a turbine engine is less than the theoretical maximum because of losses that occur along the flow path. One contributor to the losses is fluid leakage of across the tips of the compressor blades. In particular, the leakage occurs across a space between the tips of the rotating compressor blades and the surrounding stationary structure such as the casing. While minimal clearances are desired, it is critical to maintain a clearance between the blade tips and the stationary structure at all times. Tip rubbing can lead to substantial component damage, performance degradation, and extended outages.
In the past, the problem of tip clearances has been approached by initially providing large tip clearances so that the tips do not rub during non-standard engine conditions where the clearances would otherwise be expected to be the smallest because of thermal inequalities and other factors. Examples of such non-standard operating conditions include engine shut down, hot restart, spin cool, etc., all of which occur when the engine is operating at less than about 3600 rpm. However, because the minimum tip clearances are sized for these off design conditions, the clearances become overly large when the engine achieves full speed (i.e. normal operation). Consequently, the compressor/engine experiences measurable performance decreases in power and efficiency due to clearance leakage.
Other prior approaches for addressing the tip rubbing issue have included abradable coating in the blade rings and sacrificial blade tips. These approaches have shortcomings as well, for when these features rub during the first operation, the end result is still a larger tip clearance than is desired during normal operation.
Thus, there is a need for a compressor system that not only allows for larger compressor tip clearances as the engine passes through non-standard operating conditions, but also minimizes clearances during normal engine operation, thereby increasing efficiency of the compressor.